Copyright Statement

Abstract

Acacia hybrid plantations are widely planted for pulpwood in Vietnam, but have the potential to be managed for high value saw-logs. This study examined the growth and physiological responses to thinning as a management option to increase the sawlog yield and value. Treatments were: unthinned (planting density 1111 trees ha−1), thinned to 833 or 600 trees ha−1 in a single thinning at either age 2 (early) or 3 (late) years, and progressively thinned from 1111 to 833 trees ha−1 at age 2 years, and then to 600 trees ha−1 at age 3 years. Three years after early thinning to 600 stems ha−1, mean stem diameter was increased by 16.7% and the stand volume was reduced by 15.8%; thinning to 833 stems ha−1 resulted in no significant loss in stand volume and increased mean diameter by 7.5%. The leaf area index, LAI of stands thinned to 600 stems ha−1 recovered rapidly and there was no significant difference between unthinned and thinned treatments 1 year after thinning; this recovery was also associated with a reduction in litterfall. Thinning to 600 stems ha−1 increased A1500 of the lower crown by 23.5 and 17.4% at age 2 and 3 yr, respectively; and this was associated with increased foliar phosphorus concentration. Thinning reduced leaf water stress during the dry season; leaf water potential and tree growth were significantly influenced by season. It was concluded that thinning to 600 trees ha−1 at age 2 years or to 833 trees ha−1 at age 3 years will produce wood with the highest diameter.